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Study of deformation behaviour of steel arches in gate roadways
Mining Science and Technology, 3 (1985) 63-80
63
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
STUDY OF DEFORMATION BEHAVIOUR OF STEEL ARCHES IN GATE ROADWAYS S.G. Jukes Tara Mines Ltd., Knockumber, Navan, Co Meath (Ire/and)
and S.F. Smith Department of Mining Engineering, University of Nottingham, University Park, Nottingham, NG 7 2RD (U.K.) (Received October 2, 1984; accepted March 15, 1985)
ABSTRACT
Steel arches are the main form of support used in mine roadways, particularly in European coal mines. They possess special abilities in retaining significant strength and supporting properties even after substantial deformation. This paper describes a study of the types of deformation observed in gate roadways where
steel arches sustained appreciable distortion. The steel arch deformation characteristics as observed underground have been related to accompanying roadway closure and form of loading of the supports. Various patterns of gate roadway closure and steel support distortion are discussed and categories of behaviour suggested.
INTRODUCTION
ings in the same seam or adjacent seams; (b) the height of extraction and the compressibility of the pack material; (c) the strength of the strata in the immediate proximity and the effect of water on the strata; (d) proximate geology including the presence of major discontinuities and the direction of cleat; (e) mining methods, particularly comparing the drill and blast method with a machine cut profile, the degree of roadway support and
It is probably true to say that the exact mechanism of gate roadway closure is uncertain although the main factors contributing to it have been identified by Whittaker [1] and Farmer [2]. These contributing factors can be summarized as follows. (a) the stresses acting on and around the roadway resulting from redistribution of geostatic stress during excavation of coal from the longwall working or from adjacent work-
64 the treatment of the rib-side; (f) the layout of the coal face and the presence of rib-side pillars. Establishing the significance of the controlling factors is a logical approach to the problem and particularly to investigate the relative importance of each factor in turn. The complex combinations encountered in practice make this approach difficult. Despite this drawback considerable strata control knowledge has been gained by making scientifically controlled visual observations together with profile height and width measurements in gate roadways. In this paper the authors describe a method of acquiring and using information from measurements obtained whilst undertaking roadway deformation surveys. The broad assessment given by roadway deformation surveys has much to commend and is often more realistic than an isolated instrumentation site. Such sites in the middle of a length of gate roadway can produce unrepresentative results. It was with this aspect in mind that the authors undertook a series of deformation surveys in gate roadways at Cotgrave Colleiry, South Nottinghamshire Area of the National Coal Board. The intention was to acquire data relating to the relative performance of different support components and practices, and to highlight any possible weaknesses within the steel-arch roadway support system.
FIELD DATA COLLECTION
A multiple survey station method was initiated in each of the gate roadways used in this research. This method consisted of collecting data at intervals of 15 m from the face line up to a distance of 90 m out-bye, thereafter the intervals were increased to 30 m up to the 300-m mark and finally the intervals were increased to 50 m for the remainder of the roadway. The roadway support measuring system in-
volved three stages of data acquisition, which were: (a) support deformation data; (b) support distortion data; (c) site details. The arched support deformation data were acquired using the measuring scheme shown in Fig. 1 which exemplifies the booking sheet used in these roadway surveys. A base line was stretched horizontally across the roadway at a convenient level, noting the chord distance from the joint on the right-hand side, looking in-bye in all cases. This line was marked at 0.5-m intervals and at each of these, off-set measurements were taken vertically to the floor and to the inside of the arched support, thereby measuring the profile of the support and the position of the floor. The collection of arched support distortion data constituted an attempt to qualitatively describe the state of the supports in the gate roadways, such as the degree of distortion, and to attempt to correlate these data with recorded closure. Included with the data are details of the state of the support members, joints and struts. Notes were made relating to the extent and direction of deformation and failure of any support components. Typical details are also given in Fig. 1. Details of site data acquired during surveys can be categorised under the following five headings: (i) roadway size, method of formation and support; (ii) gateside packing systems; (iii) seam, height of extraction and depth of workings; (iv) face length; (v) proximate geology and roadway horizon.
DESCRIPTION OF FIELD SITES
A series of roadway surveys in maingates and tailgates were carried out at Cotgrave
6~
COLLIERY
:
COTGRAVE
DATE
DIST. FROM FACE
FACE
:
H 33's
GATE
:
TAILGATE
O
: 9/6/1982
: 750 m
CD Ln
CO
0.50
-I O
o
Measurements DEGREE OF DEFORMATION:
REMARKS:
in metres
Rib-side
leg very slightly
twisted.
Crown flattened and bent from about 0.3 m above
A
D
joint.
B
E
plastic hinge
C
F
Crown twisted on goaf-side,
G
H
(web t o r n ) 0 . 5 m a b o v e
joint bent out-of-plane
J
l.n
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bent
from
joint. Goaf-side
slightly.
Floor lift. Strut missing near hinge - inbye side. 2 missing I joint
in crown - outbye
failed
side.
in crown, others O.K.
Fig. 1. Example of roadway survey booking sheets.
Colliery following preliminary discussions with mining industry specialists on the suitability of particular sites. At the time of these surveys (1982) six faces were operative. All these faces were located in the Deep Hard Seam where the average depth was 500 m. The gate roadways servicing all
six longwall faces were surveyed. Two faces (H60's and H74's) had recently commenced production and consequently had not experienced any marked degree of deformation. Because of this, the resulting measurements have not been included in this paper although some of the data were used in the qualitative analy-
66
TABLE 1 Maingate details Maingate roadway
H11's
H33's
H37's
H72's
Pack design
Two 3.05-m wide hand built packs Rip, drill&blast
One 3.05-m wide hand built pack Conventional halfheading system. Fire rip, stable-hole, then half head 4.88 × 3.66-m arches at 0.76-m spacing 127 x 114 mm section No stilts 1.10 m + siltstone 1.19 m s a n d s t o n e / siltstone 0.76 m mudstone 1.03 m coal with dirt bands 0.26 m cannel coal 0.85 m + seatearth 1.33 m
Two 3.05-m wide hand built packs Rip, d r i l l & b l a s t
Two 3.05-m wide handbuilt packs Rip, drill& blast
4.88 x 3.66-m arches at 0.61-m spacing 127 × 114 mm section 0.76-m stilts 0.62 m + s a n d s t o n e / siltstone 0.67 m s a n d s t o n e / mudstone 1.05 m coal 0.43 m cannel coal 1.42 m + seatearth
4.88 × 3.66-m arches at 0.76-m spacing 127 x 114 mm section 0.5-m stilts 1.56 m + s i l t s t o n e / mudstone 1.07 m coal with dirt bands 0.25 m cannel coal 0.25 m coal 0.82 m + seatearth
1.5 m leaving 0.10 m of roof coal
1.5 m leaving 0.08 m of roof coal
217 m
242 m
Mining method
Roadway support and configuration Geological section
Extracted height Face length
4.88 x 3.66-m arches at 0.66-m spacing 127 x 114 mm section No stilts 1.80 m + mndstone 1.21 m coal with dirt bands 0.75 m cannel coal 1.35 m + seatearth
1.5 m leaving 0.100.12 m of roof coal and 0.4 m of cannel coal 304 m
185 m
TABLE 2 Tailgate details Tailgate roadways
H1 l ' s
H33's
H37's
H72 s
Pack design '
Two 8-20-m wide slusher filled packs Dosco Mark II roadheader
Roadway support and configuration
4.27 x 3.05-m arches at 0.76-m spacing 114 × 114 mm section No stilts
Two 3-m minimum wide bucket loader filled packs Machine ranges out roof; floor drill and blast 3.66 × 2.74-m arches at 0.91-m spacing 114 × 114 mm section 0.76-m stilts
Two 5-m wide packs
Mining method
Two 3-m minimum wide bucket loader filled packs Machine ranges out roof; floor drill and blast 3.66 x 2.74-m arches at 0.91-m spacing 114 × 114 mm section 0.76-m stilts
Geological section
1.90 m + s t i l s t o n e / mudstone 1.26 m coal with dirt bands 0.71 m cannel coal 1.41 m + seatearth
0.56 m + mudstone 0.95 m coal with dirt bands 0.42 m cannel coal 0.86 m + seatearth
Extracted height
1.5 m leaving 0.10 m of roof coal plus 0.5 m of cannel coal 304 m
1.33 m
0.62 m + s a n d s t o n e / siltstone 0.67 m s a n d s t o n e / mudstone 1.05 m coal with dirt bands 0,43 m cannel coal 1.42 m + seatearth 1.5 m leaving 0.10 m of roof coal
Face length
185 m
217m
Machine ranges out roof; floor drill and blast 3.66 × 2.74-m arches at 0.76-m spacing 1.07-m spacing for last 100 m 114 × 114 mm section 0.76-m stilts 0.85 m + mudstone siltstone 1.19 m coal with dirt bands 0.28 m cannel coal 0.18 m coal 0.30 m + seatearth 1,5 m leaving 0.80 m of roof coal
242 m
67 Figure 2 displays roadway vertical closure measurements for H37's tailgate and maingate at Cotgrave Colliery. The roadway profiles for the tailgate at the 29-, 120- and 750-m positions are illustrated in Figs. 3, 4 and 5, respectively. These show the gradual increase in closure that was experienced by the gate-road as the face advanced. As levelling was not conducted in conjunction with the surveys it was not possible to determine the contribution to closure made by floor lift. The profiles in Figs. 3-5 show the relative shape of the undeformed and deformed supports but do not show the relative positions. The positioning of the profiles is not significant since the program draws each profile around the same arbitrary baseline. Variability is a feature of many roadway closure measurements and is clearly displayed in Fig. 2. This is caused by variations in proximate geology and the effectiveness of local support resulting in differential closure. It is standard treatment to draw an average convergence profile through the points in order to clarify the closure pattern. A non-lin-
sis of arch components' distortion. The details of the maingate and tailgate roadways servicing the four other faces (H11's, H33's, H37's and H72's) are presented in Tables 1 and 2.
RESULTS
OF
UNDERGROUND
SURVEYS
The results of the underground surveys at Cotgrave Colliery together with discussions of their interpretation are presented. These deal with the quantitative closure measurements along with the qualitative arch distortion data. Upon completion of each gate roadway survey the data were treated in the same manner. A data treatment program was developed for use on a micro-computer which allowed for data analysis in the following respects: calculation of different closure types (i.e., percentage vertical closure, and percentage cross-sectional area closure), plotting of roadway closure or graphs, depicting convergence, and plotting of the roadway profile at each measured site.
O
H 37's Maingate
g
H 37's Tailgate
60
50
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Fig. 2. Vertical closure measurements for H37's gateroads.
800 (m)
I000
1200
68
Fig. 3. Support profile at 29 m out-bye in H37's tailgate. Percentage closures: vertical 16.1%, CSA 22.0%.
Fig. 5. Support profile at 750 m out-bye in H37's tailgate. Percentage closures: vertical 32.8%, CSA 46.0%.
ear curve fitting program was used to derive average closure profiles which corresponded closely to the closure measurements. Beynon [3] developed "Patternsearch" which, by altering the lines of a computer program, solves problems in non-linear curve fitting. Ascertaining the degree of fit is based upon the method of least squares in which the parameters of the curve are chosen such that the vertical distance between the data points and the curve are as small as possible.
At the time of the survey H11's was the longest coal face in the U.K. Both gate roadways experienced similar amounts of percentage closure, as shown in Fig. 6. The combination of machine-cut profile and smaller roadway dimensions in the tailgate would have suggested the likelihood of less closure than in the maingate. However, the packs in the tailgate were slusher-filled and, therefore, probably less effective in providing early support than the hand-built packs of the maingate. The lower closure values in the area 500-600 m out-bye of the coal face in both gates are due to repair operations which were taking place at the time of the surveys. Roadway repair operations had been completed in H33's maingate which made direct comparison of the closure profiles between the maingate and tailgate difficult. However, what is evident from the closure measurements in Fig. 7 is the slow development of maingate closure which is almost certainly due to the use of the half-heading system in that gate. The tailgate values indicates a very much faster increase in closure as a result of using stilts with the steel-arched supports, thereby reducing the early bearing resistance to the movement of the immediate roof strata.
Fig. 4. Support profile at 120 m out-bye in H37's tailgate. Percentage closures: vertical 22.3%, CSA 28.6%.
69 O
H ll's Maingate
60
H ll's
50
0
Tailgate
0 0
4O o O
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400
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800
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Distance Out-bye (m)
Fig. 6. Vertical closure m e a s u r e m e n t s for H l l ' s gateroads.
The higher closure values in the maingate may have resulted from the extrusion of soft floor material from beneath the solid rib pillar, thereby aggravating floor heave.
60
Both the maingate and tailgate roadways in H37's display similar closure profiles as shown in Fig. 8. The arched supports in the tailgate are smaller, and therefore stronger, but the
0
H 33's Maingate
•
H 33's Tailgate
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40
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Fig. 7. Vertical closure m e a s u r e m e n t s for H33's gateroads.
I 800
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70
H 37's Maingate 60
H 37's Tailgate 50
40 0 30
0
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Fig. 8. Vertical closure measurements for H37's gateroads.
use of a small section makes the strength more comparable to the support in the maingate. The combination of a machine-cut profile and mechanically stowed packs utilised in the tailgate appears to equate with the drill-
60
and-blast ripping and hand-built packs in the maingate in terms of overall closure results. The use of stilts in both gates appears to have encouraged the fairly rapid development of closure.
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71 The final area at Cotgrave Colliery discussed in this paper deals with H72's gateroads, and Fig. 9 depicts the closure measurements and profiles for both the maingate and tailgate. The method employed in advancing the maingate was b y drill-and-blast, whereas the face machine cut the tailgate profile. Packing techniques differed in each roadway: the maingate utilised hand-built packs, alternatively a mechanical stowing system was employed in the tailgate. Disparity in closure patterns is unlikely to be a result of these differences since they also existed on H37's face where there was little variation in closure patterns. A shorter type of stilt was utilised with the maingate roadway supports and this m a y account for the difference in closure patterns. Another feature that could well have been significant w a s a larger area of unworked coal flanking the maingate, whereas the tailgate was separated by an approximately 70-m wide pillar from a previously worked face. Whilst it is appreciated that no pillar protection was less than 1 / 1 0 depth in this situation, it is a possibility that the higher stress levels existing within the pillar may have contributed towards producing the greater levels of closure experienced by the tailgate.
A rapid rise in closure in the tailgate and a much higher overall closure (30%) in comparison to the maingate (20%) are shown in Fig. 9.
ARCH SUPPORT SUREMENTS
DISTORTION
The arch support distortion data recorded during the surveys were of a qualitative nature. At each measuring position the state of the components of the support (i.e., legs, crown, joints and struts) were recorded. This description included details of the extent and direction of distortion, occurrence of plastic hinges and failures, also the absence and state of struts. The results were tabulated in order to facilitate comparison of data. Table 3 presents a key to the symbols used in the arch distortion tables together with definitions of the descriptive categories. The results obtained during the surveys of each gate-road examined are recorded in Tables 4-11, which include details of percentage vertical and cross-sectional area closure measured at each position.
TABLE3 Keytoarch distortiontables Term/symbol
Explanation/definition
IP OP
General in-plane distortion General out-of-plane distortion Out-of-plane distortion in in-bye direction Out-of-plane distortion in out-bye direction Flattening of crown members Springing of crown members Slight distortion--up to 0.3 m movement of members - - 1 Strut missing/failed Severe distortion--greater than 0.3 m movement of members --more than 1 strut missing/failed Plastic hinge formed on member Failure of member--tearing or fracture of member
IB OB
Flat Sp ©
H
F
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76
CROWN DISTORTION
In an attempt to correlate the degree of arch support distortion and the amount of vertical closure, it was proposed that the frequency distribution of each distortion category be analysed. Clearly, the normal type of distribution is to be expected since a wide range of gate-road conditions were encountered during the surveys. The mode of the distribution is in the 28-30% vertical closure category. The lack of results b e y o n d about 55% vertical closure is owing to the fact that when such values are reached remedial work is normally performed, i.e. damaged supports are replaced or excessive floor heave is removed by dinting.
As shown in Fig. 10 some slight crown distortion appears to be universal and should be regarded as an inevitable occurrence. Such distortion, however, is not necessarily indicative of ultimate failure and collapse. The maximum relative frequency of failure in crown members recorded was 27% for the 32-34% vertical closure range. If the distribution of hinging and failure is examined two peaks become apparent: the first occurs in the 4-10% vertical closure range and the second over the 28-36% range. It is considered that these peaks of severe distortion correspond to failure of arch' supports without stilts and those utilising stilts, respectively.
0 [] []
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loo --
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8
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32
36
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Vertical closure %
Fig. 10, Frequency distribution of distortion categories for crown members.
44
48
52
56
60
60+
77
LEG DISTORTION
thereby eliminating the benefit of splayed arch legs.
Examination of the frequency distribution of distortion categories for leg members shown in Fig. 11, which include slight distortion, severe distortion, hinge formation and failure, would seem to suggest that slight deformation of leg members is almost universal. There is an increase in severity of distortion within the 20-40% closure range although the proportion of affected supports is less than one fifth. The absence of severe distortion after 40% vertical closure is accounted for by the likelihood that closure in this range can be attributed to the heaving of weak roadway floors. Consequently, rather than be deformed by this movement arch legs can actually be protected by floor heave. One prominent feature observed during these surveys was the tendency for arch legs to bow inwards after dinting, thus adopting a horse-shoe profile and
JOINT DISTORTION The occurrence of joint hinging or failure is clearly more frequent than for the arch members, such as crown and leg sections. This would suggest that the joints represent points of weakness within the support structure and that improvements in joint strength could prove beneficial. As indicated in Fig. 12, observed failures appear to be concentrated in two areas: the first is over the 28-36% vertical closure range and the second over the 44-54% range. This is possibly attributable to the influence of stilts in permitting the system to sustain higher closures before failing.
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Fig. 11. F r e q u e n c y d i s t r i b u t i o n of d i s t o r t i o n categories for leg m e m b e r s .
44
48
52
56
60
60+
78
STRUT DISTORTION
and joints). This distribution shown in Fig. 14 exhibits three peak zones. (i) the 4-12% vertical closure range where f a i l u r e / h i n g i n g of crown members predominates; (ii) the 30-36% range where all members experience about the same proportion of failure/hinging; (iii) the 45-54% range where in this zone failure/hinging of the joints is pre-eminent. It is suggested that this three-fold distribution corresponds to failure of the support system under three sets of circumstances. The first is associated with rigid support, standing on a hard floor; the second corresponds to failure of a support system utilising stilts; and thirdly, failure o f arch supports where very severe floor lift is encountered and where failure of the support is limited to the joints with other members remaining relatively unscathed [4].
The roadway deformation surveys at Cotgrave Colliery have shown that the proportion of deformed struts increases with closure, which is shown in Fig. 13, as does the severity of the distortion. The apparent lack of strut failure is attributed to the likelihood that damaged struts are removed, and therefore are classed in the survey records as "missing".
OCCURRENCE HINGES
OF
FAILURES
AND
The analysis of the arch distortion data was concluded with an examination of the distribution of failures and hinges amongst the various support components (i.e., crowns, legs
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R
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Vertical closure % Fig. 12. F r e q u e n c y d i s t r i b u t i o n of d i s t o r t i o n categories for j o i n t s .
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Vertical closure %
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Vertical closure %
Fig. 14. Frequency distribution of failure/hinging of members.
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62
80
CONCLUSIONS Generally, the analysis of roadway closure patterns is complicated by the presence of an array of contributing factors. The cases which were considered support this fact, since rarely does a sole variable alter so that its influence can be observed and assessed in a controlled manner. The surveys conducted show that both the maingates and tailgates experience comparable amounts of percentage vertical closure. This occurs despite the differences in roadway dimensions, method of packing and roadway support. The usual practice employed at Cotgrave Colliery during the period of these surveys was to use hand-built packs in the larger maingates whilst advancing using the drill-and-blast method. On the other hand, the tailgate profile was machine-cut and of a smaller dimension, with the gate-side packs mechanically stowed. Major discrepancies in closure tend to be confined to those cases where there is a marked variation in practice or where some other anomalous feature is present. In general, it appears that the use of stilts with roadway supports leads to a more rapid d e v e l o p m e n t of closure. As long as the original roadway design accounts for this, there should be no problems and the stilts give protection to the support profile. Analysis of the arch distortion data indicates that the fishplate joints of a steel arch support sustain a higher proportion of failures than any other component in the system. Peak zones of increased support distortion have been identified at certain values of percentage vertical closure. Since it was not practicable to determine the proportion of closure solely
due to the deformation of the supports, it was not possible to establish a threshold of closure after which distortion becomes excessive and failure ensues. Obviously, distortion of supports is a direct consequence of the strata loading acting on the tunnel walls. Therefore it would be more logical to correlate arch distortion with rock pressure rather than closure. In this m a n n e r it would be possible to account more accurately for the influence of proximate geology which is especially important, since the loads acting on the supports are a function of both the loading due to the overlying strata and the resistance provided by the floor material beneath the support.
ACKNOWLEDGEMENTS This work was supported by the E.C.S.C., N.C.B. and S.E.R.C. The authors express their gratitude to these organisations for their support and practical assistance given at the underground field sites. A n y views expressed are entirely those of the authors.
REFERENCES 1 B.N, Whittaker, Recording, treatment and interpretation of roadway deformation surveys, Min. Eng. London, 135 (1976) 607-667. 2 I.W. Farmer, Face and roadway stability in underground coal mines: geotechnical criteria, Report to NCB, Research Contract 7220-AC/806, University of Newcastle Upon Tyne, Dec. 1980. 3 R.J. Beynon, Patternsearch: non-linear curve fitting program for Apple II, Department of Biochemistry, University of Liverpool, 1981. 4 S.G. Jukes, An investigation into steel arch support characteristics of mining tunnels, Ph.D. Thesis, University of Nottingham, Oct. 1983.
63
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
STUDY OF DEFORMATION BEHAVIOUR OF STEEL ARCHES IN GATE ROADWAYS S.G. Jukes Tara Mines Ltd., Knockumber, Navan, Co Meath (Ire/and)
and S.F. Smith Department of Mining Engineering, University of Nottingham, University Park, Nottingham, NG 7 2RD (U.K.) (Received October 2, 1984; accepted March 15, 1985)
ABSTRACT
Steel arches are the main form of support used in mine roadways, particularly in European coal mines. They possess special abilities in retaining significant strength and supporting properties even after substantial deformation. This paper describes a study of the types of deformation observed in gate roadways where
steel arches sustained appreciable distortion. The steel arch deformation characteristics as observed underground have been related to accompanying roadway closure and form of loading of the supports. Various patterns of gate roadway closure and steel support distortion are discussed and categories of behaviour suggested.
INTRODUCTION
ings in the same seam or adjacent seams; (b) the height of extraction and the compressibility of the pack material; (c) the strength of the strata in the immediate proximity and the effect of water on the strata; (d) proximate geology including the presence of major discontinuities and the direction of cleat; (e) mining methods, particularly comparing the drill and blast method with a machine cut profile, the degree of roadway support and
It is probably true to say that the exact mechanism of gate roadway closure is uncertain although the main factors contributing to it have been identified by Whittaker [1] and Farmer [2]. These contributing factors can be summarized as follows. (a) the stresses acting on and around the roadway resulting from redistribution of geostatic stress during excavation of coal from the longwall working or from adjacent work-
64 the treatment of the rib-side; (f) the layout of the coal face and the presence of rib-side pillars. Establishing the significance of the controlling factors is a logical approach to the problem and particularly to investigate the relative importance of each factor in turn. The complex combinations encountered in practice make this approach difficult. Despite this drawback considerable strata control knowledge has been gained by making scientifically controlled visual observations together with profile height and width measurements in gate roadways. In this paper the authors describe a method of acquiring and using information from measurements obtained whilst undertaking roadway deformation surveys. The broad assessment given by roadway deformation surveys has much to commend and is often more realistic than an isolated instrumentation site. Such sites in the middle of a length of gate roadway can produce unrepresentative results. It was with this aspect in mind that the authors undertook a series of deformation surveys in gate roadways at Cotgrave Colleiry, South Nottinghamshire Area of the National Coal Board. The intention was to acquire data relating to the relative performance of different support components and practices, and to highlight any possible weaknesses within the steel-arch roadway support system.
FIELD DATA COLLECTION
A multiple survey station method was initiated in each of the gate roadways used in this research. This method consisted of collecting data at intervals of 15 m from the face line up to a distance of 90 m out-bye, thereafter the intervals were increased to 30 m up to the 300-m mark and finally the intervals were increased to 50 m for the remainder of the roadway. The roadway support measuring system in-
volved three stages of data acquisition, which were: (a) support deformation data; (b) support distortion data; (c) site details. The arched support deformation data were acquired using the measuring scheme shown in Fig. 1 which exemplifies the booking sheet used in these roadway surveys. A base line was stretched horizontally across the roadway at a convenient level, noting the chord distance from the joint on the right-hand side, looking in-bye in all cases. This line was marked at 0.5-m intervals and at each of these, off-set measurements were taken vertically to the floor and to the inside of the arched support, thereby measuring the profile of the support and the position of the floor. The collection of arched support distortion data constituted an attempt to qualitatively describe the state of the supports in the gate roadways, such as the degree of distortion, and to attempt to correlate these data with recorded closure. Included with the data are details of the state of the support members, joints and struts. Notes were made relating to the extent and direction of deformation and failure of any support components. Typical details are also given in Fig. 1. Details of site data acquired during surveys can be categorised under the following five headings: (i) roadway size, method of formation and support; (ii) gateside packing systems; (iii) seam, height of extraction and depth of workings; (iv) face length; (v) proximate geology and roadway horizon.
DESCRIPTION OF FIELD SITES
A series of roadway surveys in maingates and tailgates were carried out at Cotgrave
6~
COLLIERY
:
COTGRAVE
DATE
DIST. FROM FACE
FACE
:
H 33's
GATE
:
TAILGATE
O
: 9/6/1982
: 750 m
CD Ln
CO
0.50
-I O
o
Measurements DEGREE OF DEFORMATION:
REMARKS:
in metres
Rib-side
leg very slightly
twisted.
Crown flattened and bent from about 0.3 m above
A
D
joint.
B
E
plastic hinge
C
F
Crown twisted on goaf-side,
G
H
(web t o r n ) 0 . 5 m a b o v e
joint bent out-of-plane
J
l.n
T-
bent
from
joint. Goaf-side
slightly.
Floor lift. Strut missing near hinge - inbye side. 2 missing I joint
in crown - outbye
failed
side.
in crown, others O.K.
Fig. 1. Example of roadway survey booking sheets.
Colliery following preliminary discussions with mining industry specialists on the suitability of particular sites. At the time of these surveys (1982) six faces were operative. All these faces were located in the Deep Hard Seam where the average depth was 500 m. The gate roadways servicing all
six longwall faces were surveyed. Two faces (H60's and H74's) had recently commenced production and consequently had not experienced any marked degree of deformation. Because of this, the resulting measurements have not been included in this paper although some of the data were used in the qualitative analy-
66
TABLE 1 Maingate details Maingate roadway
H11's
H33's
H37's
H72's
Pack design
Two 3.05-m wide hand built packs Rip, drill&blast
One 3.05-m wide hand built pack Conventional halfheading system. Fire rip, stable-hole, then half head 4.88 × 3.66-m arches at 0.76-m spacing 127 x 114 mm section No stilts 1.10 m + siltstone 1.19 m s a n d s t o n e / siltstone 0.76 m mudstone 1.03 m coal with dirt bands 0.26 m cannel coal 0.85 m + seatearth 1.33 m
Two 3.05-m wide hand built packs Rip, d r i l l & b l a s t
Two 3.05-m wide handbuilt packs Rip, drill& blast
4.88 x 3.66-m arches at 0.61-m spacing 127 × 114 mm section 0.76-m stilts 0.62 m + s a n d s t o n e / siltstone 0.67 m s a n d s t o n e / mudstone 1.05 m coal 0.43 m cannel coal 1.42 m + seatearth
4.88 × 3.66-m arches at 0.76-m spacing 127 x 114 mm section 0.5-m stilts 1.56 m + s i l t s t o n e / mudstone 1.07 m coal with dirt bands 0.25 m cannel coal 0.25 m coal 0.82 m + seatearth
1.5 m leaving 0.10 m of roof coal
1.5 m leaving 0.08 m of roof coal
217 m
242 m
Mining method
Roadway support and configuration Geological section
Extracted height Face length
4.88 x 3.66-m arches at 0.66-m spacing 127 x 114 mm section No stilts 1.80 m + mndstone 1.21 m coal with dirt bands 0.75 m cannel coal 1.35 m + seatearth
1.5 m leaving 0.100.12 m of roof coal and 0.4 m of cannel coal 304 m
185 m
TABLE 2 Tailgate details Tailgate roadways
H1 l ' s
H33's
H37's
H72 s
Pack design '
Two 8-20-m wide slusher filled packs Dosco Mark II roadheader
Roadway support and configuration
4.27 x 3.05-m arches at 0.76-m spacing 114 × 114 mm section No stilts
Two 3-m minimum wide bucket loader filled packs Machine ranges out roof; floor drill and blast 3.66 × 2.74-m arches at 0.91-m spacing 114 × 114 mm section 0.76-m stilts
Two 5-m wide packs
Mining method
Two 3-m minimum wide bucket loader filled packs Machine ranges out roof; floor drill and blast 3.66 x 2.74-m arches at 0.91-m spacing 114 × 114 mm section 0.76-m stilts
Geological section
1.90 m + s t i l s t o n e / mudstone 1.26 m coal with dirt bands 0.71 m cannel coal 1.41 m + seatearth
0.56 m + mudstone 0.95 m coal with dirt bands 0.42 m cannel coal 0.86 m + seatearth
Extracted height
1.5 m leaving 0.10 m of roof coal plus 0.5 m of cannel coal 304 m
1.33 m
0.62 m + s a n d s t o n e / siltstone 0.67 m s a n d s t o n e / mudstone 1.05 m coal with dirt bands 0,43 m cannel coal 1.42 m + seatearth 1.5 m leaving 0.10 m of roof coal
Face length
185 m
217m
Machine ranges out roof; floor drill and blast 3.66 × 2.74-m arches at 0.76-m spacing 1.07-m spacing for last 100 m 114 × 114 mm section 0.76-m stilts 0.85 m + mudstone siltstone 1.19 m coal with dirt bands 0.28 m cannel coal 0.18 m coal 0.30 m + seatearth 1,5 m leaving 0.80 m of roof coal
242 m
67 Figure 2 displays roadway vertical closure measurements for H37's tailgate and maingate at Cotgrave Colliery. The roadway profiles for the tailgate at the 29-, 120- and 750-m positions are illustrated in Figs. 3, 4 and 5, respectively. These show the gradual increase in closure that was experienced by the gate-road as the face advanced. As levelling was not conducted in conjunction with the surveys it was not possible to determine the contribution to closure made by floor lift. The profiles in Figs. 3-5 show the relative shape of the undeformed and deformed supports but do not show the relative positions. The positioning of the profiles is not significant since the program draws each profile around the same arbitrary baseline. Variability is a feature of many roadway closure measurements and is clearly displayed in Fig. 2. This is caused by variations in proximate geology and the effectiveness of local support resulting in differential closure. It is standard treatment to draw an average convergence profile through the points in order to clarify the closure pattern. A non-lin-
sis of arch components' distortion. The details of the maingate and tailgate roadways servicing the four other faces (H11's, H33's, H37's and H72's) are presented in Tables 1 and 2.
RESULTS
OF
UNDERGROUND
SURVEYS
The results of the underground surveys at Cotgrave Colliery together with discussions of their interpretation are presented. These deal with the quantitative closure measurements along with the qualitative arch distortion data. Upon completion of each gate roadway survey the data were treated in the same manner. A data treatment program was developed for use on a micro-computer which allowed for data analysis in the following respects: calculation of different closure types (i.e., percentage vertical closure, and percentage cross-sectional area closure), plotting of roadway closure or graphs, depicting convergence, and plotting of the roadway profile at each measured site.
O
H 37's Maingate
g
H 37's Tailgate
60
50
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O 0
o >
30
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Fig. 2. Vertical closure measurements for H37's gateroads.
800 (m)
I000
1200
68
Fig. 3. Support profile at 29 m out-bye in H37's tailgate. Percentage closures: vertical 16.1%, CSA 22.0%.
Fig. 5. Support profile at 750 m out-bye in H37's tailgate. Percentage closures: vertical 32.8%, CSA 46.0%.
ear curve fitting program was used to derive average closure profiles which corresponded closely to the closure measurements. Beynon [3] developed "Patternsearch" which, by altering the lines of a computer program, solves problems in non-linear curve fitting. Ascertaining the degree of fit is based upon the method of least squares in which the parameters of the curve are chosen such that the vertical distance between the data points and the curve are as small as possible.
At the time of the survey H11's was the longest coal face in the U.K. Both gate roadways experienced similar amounts of percentage closure, as shown in Fig. 6. The combination of machine-cut profile and smaller roadway dimensions in the tailgate would have suggested the likelihood of less closure than in the maingate. However, the packs in the tailgate were slusher-filled and, therefore, probably less effective in providing early support than the hand-built packs of the maingate. The lower closure values in the area 500-600 m out-bye of the coal face in both gates are due to repair operations which were taking place at the time of the surveys. Roadway repair operations had been completed in H33's maingate which made direct comparison of the closure profiles between the maingate and tailgate difficult. However, what is evident from the closure measurements in Fig. 7 is the slow development of maingate closure which is almost certainly due to the use of the half-heading system in that gate. The tailgate values indicates a very much faster increase in closure as a result of using stilts with the steel-arched supports, thereby reducing the early bearing resistance to the movement of the immediate roof strata.
Fig. 4. Support profile at 120 m out-bye in H37's tailgate. Percentage closures: vertical 22.3%, CSA 28.6%.
69 O
H ll's Maingate
60
H ll's
50
0
Tailgate
0 0
4O o O
%
30 /
~.
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•
20
O 1o
I
I
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200
400
600
I
,,,,h ,,
800
I000
Distance Out-bye (m)
Fig. 6. Vertical closure m e a s u r e m e n t s for H l l ' s gateroads.
The higher closure values in the maingate may have resulted from the extrusion of soft floor material from beneath the solid rib pillar, thereby aggravating floor heave.
60
Both the maingate and tailgate roadways in H37's display similar closure profiles as shown in Fig. 8. The arched supports in the tailgate are smaller, and therefore stronger, but the
0
H 33's Maingate
•
H 33's Tailgate
50 0 @ 0
40
o
0
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O@
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Fig. 7. Vertical closure m e a s u r e m e n t s for H33's gateroads.
I 800
I
1000
70
H 37's Maingate 60
H 37's Tailgate 50
40 0 30
0
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a
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800
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Fig. 8. Vertical closure measurements for H37's gateroads.
use of a small section makes the strength more comparable to the support in the maingate. The combination of a machine-cut profile and mechanically stowed packs utilised in the tailgate appears to equate with the drill-
60
and-blast ripping and hand-built packs in the maingate in terms of overall closure results. The use of stilts in both gates appears to have encouraged the fairly rapid development of closure.
O
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•
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50 4O o
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Fig. 9. Vertical closure measurements for H72's gateroads.
i
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71 The final area at Cotgrave Colliery discussed in this paper deals with H72's gateroads, and Fig. 9 depicts the closure measurements and profiles for both the maingate and tailgate. The method employed in advancing the maingate was b y drill-and-blast, whereas the face machine cut the tailgate profile. Packing techniques differed in each roadway: the maingate utilised hand-built packs, alternatively a mechanical stowing system was employed in the tailgate. Disparity in closure patterns is unlikely to be a result of these differences since they also existed on H37's face where there was little variation in closure patterns. A shorter type of stilt was utilised with the maingate roadway supports and this m a y account for the difference in closure patterns. Another feature that could well have been significant w a s a larger area of unworked coal flanking the maingate, whereas the tailgate was separated by an approximately 70-m wide pillar from a previously worked face. Whilst it is appreciated that no pillar protection was less than 1 / 1 0 depth in this situation, it is a possibility that the higher stress levels existing within the pillar may have contributed towards producing the greater levels of closure experienced by the tailgate.
A rapid rise in closure in the tailgate and a much higher overall closure (30%) in comparison to the maingate (20%) are shown in Fig. 9.
ARCH SUPPORT SUREMENTS
DISTORTION
The arch support distortion data recorded during the surveys were of a qualitative nature. At each measuring position the state of the components of the support (i.e., legs, crown, joints and struts) were recorded. This description included details of the extent and direction of distortion, occurrence of plastic hinges and failures, also the absence and state of struts. The results were tabulated in order to facilitate comparison of data. Table 3 presents a key to the symbols used in the arch distortion tables together with definitions of the descriptive categories. The results obtained during the surveys of each gate-road examined are recorded in Tables 4-11, which include details of percentage vertical and cross-sectional area closure measured at each position.
TABLE3 Keytoarch distortiontables Term/symbol
Explanation/definition
IP OP
General in-plane distortion General out-of-plane distortion Out-of-plane distortion in in-bye direction Out-of-plane distortion in out-bye direction Flattening of crown members Springing of crown members Slight distortion--up to 0.3 m movement of members - - 1 Strut missing/failed Severe distortion--greater than 0.3 m movement of members --more than 1 strut missing/failed Plastic hinge formed on member Failure of member--tearing or fracture of member
IB OB
Flat Sp ©
H
F
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76
CROWN DISTORTION
In an attempt to correlate the degree of arch support distortion and the amount of vertical closure, it was proposed that the frequency distribution of each distortion category be analysed. Clearly, the normal type of distribution is to be expected since a wide range of gate-road conditions were encountered during the surveys. The mode of the distribution is in the 28-30% vertical closure category. The lack of results b e y o n d about 55% vertical closure is owing to the fact that when such values are reached remedial work is normally performed, i.e. damaged supports are replaced or excessive floor heave is removed by dinting.
As shown in Fig. 10 some slight crown distortion appears to be universal and should be regarded as an inevitable occurrence. Such distortion, however, is not necessarily indicative of ultimate failure and collapse. The maximum relative frequency of failure in crown members recorded was 27% for the 32-34% vertical closure range. If the distribution of hinging and failure is examined two peaks become apparent: the first occurs in the 4-10% vertical closure range and the second over the 28-36% range. It is considered that these peaks of severe distortion correspond to failure of arch' supports without stilts and those utilising stilts, respectively.
0 [] []
Hinge Forming
M
Failed
Slight Severe
loo --
8o d •
6o
M
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4o
2o o 4
8
12
16
20
24
28
32
36
40
Vertical closure %
Fig. 10, Frequency distribution of distortion categories for crown members.
44
48
52
56
60
60+
77
LEG DISTORTION
thereby eliminating the benefit of splayed arch legs.
Examination of the frequency distribution of distortion categories for leg members shown in Fig. 11, which include slight distortion, severe distortion, hinge formation and failure, would seem to suggest that slight deformation of leg members is almost universal. There is an increase in severity of distortion within the 20-40% closure range although the proportion of affected supports is less than one fifth. The absence of severe distortion after 40% vertical closure is accounted for by the likelihood that closure in this range can be attributed to the heaving of weak roadway floors. Consequently, rather than be deformed by this movement arch legs can actually be protected by floor heave. One prominent feature observed during these surveys was the tendency for arch legs to bow inwards after dinting, thus adopting a horse-shoe profile and
JOINT DISTORTION The occurrence of joint hinging or failure is clearly more frequent than for the arch members, such as crown and leg sections. This would suggest that the joints represent points of weakness within the support structure and that improvements in joint strength could prove beneficial. As indicated in Fig. 12, observed failures appear to be concentrated in two areas: the first is over the 28-36% vertical closure range and the second over the 44-54% range. This is possibly attributable to the influence of stilts in permitting the system to sustain higher closures before failing.
[]
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[]
Hinge Forming Failed
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Fig. 11. F r e q u e n c y d i s t r i b u t i o n of d i s t o r t i o n categories for leg m e m b e r s .
44
48
52
56
60
60+
78
STRUT DISTORTION
and joints). This distribution shown in Fig. 14 exhibits three peak zones. (i) the 4-12% vertical closure range where f a i l u r e / h i n g i n g of crown members predominates; (ii) the 30-36% range where all members experience about the same proportion of failure/hinging; (iii) the 45-54% range where in this zone failure/hinging of the joints is pre-eminent. It is suggested that this three-fold distribution corresponds to failure of the support system under three sets of circumstances. The first is associated with rigid support, standing on a hard floor; the second corresponds to failure of a support system utilising stilts; and thirdly, failure o f arch supports where very severe floor lift is encountered and where failure of the support is limited to the joints with other members remaining relatively unscathed [4].
The roadway deformation surveys at Cotgrave Colliery have shown that the proportion of deformed struts increases with closure, which is shown in Fig. 13, as does the severity of the distortion. The apparent lack of strut failure is attributed to the likelihood that damaged struts are removed, and therefore are classed in the survey records as "missing".
OCCURRENCE HINGES
OF
FAILURES
AND
The analysis of the arch distortion data was concluded with an examination of the distribution of failures and hinges amongst the various support components (i.e., crowns, legs
0
Slight Severe Hinge Forming
R
Failed
~2
1oo
8o
60
=
--
4o 20
0
8
12
16
20
24
28
32
36
Vertical closure % Fig. 12. F r e q u e n c y d i s t r i b u t i o n of d i s t o r t i o n categories for j o i n t s .
40
44
48
52
I_ 56
60
60+
79
eo .a
[]
Slight
]
Severe
100
I
80
60
---2-----
g
40
V---
g
lJ
---4
f.
-i
20
0
0
4
8
12
16
20
24
28
l* /.
I ,/I/ 32
36
11 II II i,
40
44
48
,
52
J
56
i
t
60
i
60+
Vertical closure %
Fig. 13. Frequency distribution of distortion categories for struts.
[]
Crown
80
Leg m 70
:
60
50
~
40 Jo i i
20 ,--i
m m 0
4
8
12
16
20
24
28
32
36
40
Vertical closure %
Fig. 14. Frequency distribution of failure/hinging of members.
44
48
52
56
60
62
80
CONCLUSIONS Generally, the analysis of roadway closure patterns is complicated by the presence of an array of contributing factors. The cases which were considered support this fact, since rarely does a sole variable alter so that its influence can be observed and assessed in a controlled manner. The surveys conducted show that both the maingates and tailgates experience comparable amounts of percentage vertical closure. This occurs despite the differences in roadway dimensions, method of packing and roadway support. The usual practice employed at Cotgrave Colliery during the period of these surveys was to use hand-built packs in the larger maingates whilst advancing using the drill-and-blast method. On the other hand, the tailgate profile was machine-cut and of a smaller dimension, with the gate-side packs mechanically stowed. Major discrepancies in closure tend to be confined to those cases where there is a marked variation in practice or where some other anomalous feature is present. In general, it appears that the use of stilts with roadway supports leads to a more rapid d e v e l o p m e n t of closure. As long as the original roadway design accounts for this, there should be no problems and the stilts give protection to the support profile. Analysis of the arch distortion data indicates that the fishplate joints of a steel arch support sustain a higher proportion of failures than any other component in the system. Peak zones of increased support distortion have been identified at certain values of percentage vertical closure. Since it was not practicable to determine the proportion of closure solely
due to the deformation of the supports, it was not possible to establish a threshold of closure after which distortion becomes excessive and failure ensues. Obviously, distortion of supports is a direct consequence of the strata loading acting on the tunnel walls. Therefore it would be more logical to correlate arch distortion with rock pressure rather than closure. In this m a n n e r it would be possible to account more accurately for the influence of proximate geology which is especially important, since the loads acting on the supports are a function of both the loading due to the overlying strata and the resistance provided by the floor material beneath the support.
ACKNOWLEDGEMENTS This work was supported by the E.C.S.C., N.C.B. and S.E.R.C. The authors express their gratitude to these organisations for their support and practical assistance given at the underground field sites. A n y views expressed are entirely those of the authors.
REFERENCES 1 B.N, Whittaker, Recording, treatment and interpretation of roadway deformation surveys, Min. Eng. London, 135 (1976) 607-667. 2 I.W. Farmer, Face and roadway stability in underground coal mines: geotechnical criteria, Report to NCB, Research Contract 7220-AC/806, University of Newcastle Upon Tyne, Dec. 1980. 3 R.J. Beynon, Patternsearch: non-linear curve fitting program for Apple II, Department of Biochemistry, University of Liverpool, 1981. 4 S.G. Jukes, An investigation into steel arch support characteristics of mining tunnels, Ph.D. Thesis, University of Nottingham, Oct. 1983.